When I ankle carry, I carry my 642 in a Renegade Cozy Partner 500. Best ankle rig made IMO. Anyway, I hate it when people say "I THINK X would be too heavy or whatever to ankle carry." If you've never tried it, it doesn't make much of a judgement. I'm tempted to get a Renegade for my 26 and give it a go. Yes heavier but also double capacity.
With that said, I would still probably run the 642.
S&W out of the two mentioned. For me personally, I've only ever ankle-carried a p3at as a BUG in a generic holster. It wasn't comfortable. I would assume like "regular carry" the holster makes all the difference. And would probable carry the p3at ankle, and have my S&W 442 in my RF pocket, wich is my EDC anyway.